Investors are thrilled about Cliffs' (CLF+17.8%) better-than-expected Q1, but Cowen questions whether the miner has turned the corner. CLF is a relatively high-cost producer, Cowen says, so weakness in iron ore prices should continue to have an outsized impact on shares. The firm notes that tax rate variance was meaningful in Q1; utilizing a 30% rate would have brought adjusted EPS to $0.37 vs. reported $0.60.